I believe the your calculation is working correctly - but it appears that the static categories are beign compared against a blank field in either the Sept column or the Oct column. I may be missing something, but I believe this is likely why there are so many 'static' results. Those blank fields cause it to fall into the final 'else' logic of your calc.
Thanks for your response. You are correct in that the problem is caused where there is blank data for a customer in either September or October. What I don't understand is why the ZN function doesn't stop this from happening.
The table calc (using ZN) works in so far as the calculation to show the difference between the two months calculates correctly and this can be seen in the 2nd sheet - when it comes to the 'difference' every customer has an entry.
It would appear that the IF calc is running ahead of the table calc which it is why it fails. What I would like to be able to find out is how to change this behaviour so it will calculate in the way I am after OR if someone could suggest another method entirely. I am not wedded to the route, it's the destination that counts.